A fuel cell is an energy conversion device that electrochemically reacts a fuel with an oxidant to generate direct current (DC) power. A fuel cell typically consists of an anode, an electrolyte material and a cathode. In a Polymer Electrolyte Membrane (PEM) fuel cell, the anode and the cathode are bonded onto the polymer electrolyte material to form an individual cell. The individual cell generates a relatively small voltage, typically 0.6-0.7 Volts, but may produce high currents. To achieve higher voltages that are practically useful, electrical interconnects are used to connect a relatively large number of individual cells in series. The electrical interconnect also usually provides passageways which allow the flow of fuel to the anode of each cell, oxidant to the cathode and water for cooling and humidification purposes. These electrical interconnects are commonly referred to as “plates”.
The term “plates”, in the present field, incorporates a number of different types of plates, such as end plates, buss plates, fuel/oxidant plates, fuel/cooling (air or liquid) plates and oxidant/cooling (air or liquid) plates. The buss plates are used to collect current from the active area of a fuel cell, and have an extended conductive tab that allows an electrical connection for supplying a load. Typically, two buss plates, located at each end of the active section of the fuel cell stack, are used in a fuel cell stack. The assembly of fuel cells thus formed is referred to as a fuel cell stack.
A fuel cell power generation system typically comprises a fuel cell stack that consists of a humidification section and an electrochemically active section although some systems are designed such that humidification of the reactants occurs outside of the fuel cell stack. The humidification section imparts water vapour to the hydrogen containing fuel stream and the oxygen containing oxidant stream that are fed into the fuel cell stack. The electrochemically active section comprises fuel cells for promoting the electrocatalytic conversion of the humidified fuel and oxidant streams to electric current and product water. The electrochemically active section also includes a coolant water stream for absorbing heat generated in the active section. The fuel cell system includes a heat exchanger for removing heat from the coolant water stream exiting the active section, a water separator for removing water from the oxidant stream exiting the fuel cell stack, and a coolant reservoir for receiving the removed water stream from the water separator and from the heat exchanger. The coolant water stream is drawn from the coolant reservoir. An example of the design of such an Integrated Fuel Cell Power Generation System is disclosed in U.S. Pat. No. 5,200,278. In other designs, air is used as the coolant instead of water.
The current art of connecting a plurality of individual fuel cells in series to achieve higher voltages leads to the disadvantage that all of the connected fuel cells can be rendered inoperable if one of the fuel cells in the fuel cell stack fails. The probability of failure of a single fuel cell in a stack is not known, but for a given probability of failure, the weakest link theory allows estimation of the probability of failure for a stack that is connected in series. Assume, for example, that a 1 kW stack that consists of 50 fuel cells in series and that each cell has the same probability of failure. If the failure probability for each cell is 0.01over a specified period, then the probability of survival is 0.99. The probability of having a good stack is then 0.9950 or about 0.60. The probability of failure over the specified period therefore is about 40%, a very high value. Reliability of fuel cell stacks is a major concern for many applications.
While the fuel used in low temperature fuel cells may be pure hydrogen, commonly an impure hydrogen stream is used as the fuel source. For example, impure hydrogen produced by reforming hydrocarbon or oxygenated hydrocarbons such as natural gas, propane, or methanol may be used. These impure hydrogen streams commonly contain significant amounts of electrocatalyst poisons such as carbon monoxide, which seriously degrade the power output of the fuel cell stack. Similarly, electrocatalyst poisons can be introduced through the oxidant stream, especially when air is used as the oxidant. Poisoning of fuel cell stacks is another major concern for many applications.
In the prior art, the following patents U.S. Pat. Nos. 6,339,313 and 6,541,941, of the same assignee, disclose a means for alleviating both the reliability and poisoning problems outlined above. It is well known that electrocatalyst poisons can be removed from the anode by periodically raising the anode potential and that electrocatalyst poisons can be removed from the cathode by periodically lowering the cathode potential. For example, carbon monoxide can be removed from a platinum electrocatalyst by raising the anode potential to approximately 700 mV vs. a Reference Hydrogen Electrode (RHE). At this potential the carbon monoxide (CO) is oxidized to carbon dioxide (CO2), which is released into the fuel stream. The same patents also disclose that the same devices and methods can be used to supplement a weak cell or by-pass a defective cell in a stack to thereby increase the overall stack reliability. In order to effect poison removal or cell supplementation or by-pass, very large currents which may be equal to or even exceed the maximum stack current must be introduced into the edge of the cell plates and this current must then flow in the plane of the plate to be equally distributed over the surface of the individual cell that is being treated.
Presently, the fuel cell plates utilized in PEM fuel cells are commonly made of machined graphite, moulded composite graphite/plastic materials or an inexpensive, flexible sheet material (such as Grafoil™) into which reactant flow fields are pressed. These materials have the advantage that they are chemically stable in the harsh operating environment encountered in a PEM fuel cell and they have a sufficiently high electrical conductivity such that the voltage drop caused by the flow of the stack current through the thin plate (i.e., normal to the plate surface) is sufficiently small.
In FIG. 1 of the prior art, a fuel cell stack assembly 10 is shown. The fuel cell stack assembly 10 includes an electrochemically active section 26 and a humidification section 28. The stack assembly 10 is a modular plate and frame design, and includes a compression end plate 16 and a fluid end plate 18. An optional pneumatic piston 17, positioned within compression end plate 16, applies uniform pressure to the assembly to promote sealing. Buss plates 22 and 24 located on opposite ends of active section 26 provide the negative and positive contacts, respectively, to draw current generated by the fuel cell stack assembly 10 to a load (not shown). Tie rods 20 extend between end plates 16 and 18 to retain and secure stack assembly 10 in its assembled state with fastening nuts 21.
The active section 26 includes, in addition to buss plates 22 and 24, a plurality of fuel cell assemblies 12, each assembly consisting of two fuel cells. The humidification section 28 includes a plurality of humidification assemblies 14, each assembly consisting of a fuel or oxidant reactant flow field plate, a water flow field plate and a water vapor transport membrane interposed between the reactant flow field plate and the water flow field plate. The humidification section 28 imparts water vapor to the fuel and oxidant streams that are later fed to the active section 26, thereby preventing the membranes within the active section from drying out.
FIG. 2, of the prior art, is a sectional view of the fuel cell assemblies 12, which constitute the electrochemically active section of fuel cell stack assembly 10 of FIG. 1. In particular, assembly 12 includes graphite flow field plates 42, 44 and 54. Fuel flow field channels 54b and 44b are engraved or milled into plates 44 and 54 respectively as shown. Oxidant flow field channels 42a and 44a are engraved or milled into plates 42 and 44, respectively, as shown. Water flow field channels 42b are engraved or milled into plate 42 on the side opposite channels 42a, as shown. The membrane electrode assemblies 48 are interposed between fuel flow field channels 44b and oxidant flow field channels 42a and between fuel flow field channels 54b and oxidant flow field channels 44a. 
Membrane electrode assemblies 48 are essentially identical. Each membrane electrode assembly 48 comprises two layers of porous electrically conductive sheet material, preferably carbon fiber paper, and a solid polymer electrolyte or ion exchange membrane interposed between the two layers of porous electrically conductive sheet material. The sheet material layers are each coated with catalyst, preferably finely divided platinum, on the surfaces adjacent and in contact with the ion exchange membrane to render the sheet material electrochemically active. The two electrodes and ion exchange membrane are heat and pressure consolidated to form membrane electrode assemblies 48.
The existing fuel cell plate designs have electrical and/or physical deficiencies that make connection of the Fuel Cell Health Manager (FCHM) systems disclosed in U.S. Pat. Nos. 6,339,313 and 6,541,941, difficult or impossible. The electrical conductivity of pure graphite is relatively low being of the order of 800×10−6/ohm-cm and that of moulded graphite/plastic plates or Grafoil™ several times lower than pure graphite. With typical electrode currents in PEM fuel cell stacks being of the order of 1 A/cm2, it can easily be shown that unacceptably large voltage drops (several hundreds of mV) will occur when currents are introduced into the edge of the plates in high power stacks. Such large voltage drops will result in uneven distribution of current through the plates and ineffective removal of fuel cell poisons. Existing fuel cell plates also fail to provide the extensions that are needed for electrical connection of the FCHM to the stack and for heat removal from high power FCHM electronic components such as Metal Oxide Field Effect Transistors (MOSFETs).
In view of the above-noted shortcomings, it is therefore desirable to provide an improved electrical interconnect for use in a fuel cell stack which is highly conductive and chemically durable, and suitable for use with a management and control tool such as the FCHM. Furthermore, there is a need for an improved apparatus and method for electrically connecting the FCHM and control systems to fuel cell stacks. In particular, there is a need for an electrical interconnect that has high electrical conductivity so that there is an acceptable level of voltage drop across the plate during poison removal, also termed rejuvenation, and/or cell supplementation and by-pass, that is also chemically stable in the fuel cell operating environment and that can be used as a heat sink or have a heat sink attached for the high power electronic components associated with the FCHM system. In addition, there is a need for an apparatus and method that maintains the benefit of reduced overall size to maintain a high overall power density of the fuel cell system.